The Calcium ion is one of elements having a highest content in the body, which exists in the bone structure, cells and body liquid to take part in neurotransmission and muscle contraction and also is involved in wide physiological actions such as by delivering various stimuli such as growth factors or hormones to cells. Most of calcium in a human body exists in bones and teeth in the form of calcium phosphate. However, with advancing years, the absorption rate of calcium decreases, whereby calcium in the bone escapes, causing bone-related diseases such as osteoporosis and osteomalacia.
The calcium absorption in the digestive organs is known to depend on the state of calcium per se in an aqueous solution. Thus, in the stomach and intestines having a low pH, calcium is maintained in the ion state so that it can be in large part absorbed, while in the lower small intestines where the pH is near neutrality, it forms an insoluble salt with phosphorous which can not absorbed and thereby is excreted out of the body. Excessive intake of such insoluble calcium is known to cause a calculus.
It has been reported that whey calcium present in milk has the highest absorption rate of food. This is believed because lactose in the milk may promote the absorption of the whey calcium. However, it is recently proposed that the highest absorption rate of the whey calcium is because phosphopeptide which is an enzymatic hydrolysate of whey protein inhibits formation of calcium phosphate in the digestive organs of the human body. It is known that the phosphopeptide is not digested by digestive enzymes but forms water-soluble complex with calcium ion.
Manson, et al. separated phosphopeptide composed of 25 amino acids from enzymatic hydrolysates of β-casein and showed that this phosphopeptide comprising 4 phosphoserines (Arch. Biochem. Biophys. 145:16–26). Thus, it was discovered that the phosphate group (PO43−) attached to serine residue of phosphopeptide forms the water soluble complex with calcium in the digestive organs and was thereby assumed that the increase in solubility of calcium by casein phosphopeptide is attributable to the phosphate group of the phosphopeptide. Also, according to various animal tests, it was shown that phosphopeptide increases the amount of soluble calcium, thereby promoting the absorption thereof (J. Nutr. 110:2141–2148, Br. J. Nutr. 49:67–76). Sato, et al. in Japan found that casein phosphopeptide can increase the calcium absorption in the small intestines of rats via in vitro experiment (J. Nutr. Sci. Vitaminol. 32:67–76).
Phosphorylation of casein phosphopeptide existing in milk is a reaction which specifically occurs by casein kinases in golgi complex after casein passes through the membrane of endoplasmic reticulum (J. Dairy. Sci. 71:324–336). Up to date, it has been shown that the casein phosphopeptide in milk among natural food can most effectively promote the calcium absorption to the body. However, since the casein phosphopeptide is extracted from milk, there is a problem in that the production amount is limited.
Therefore, there is demand for new and stable food and medical material which has an increased binding ability to calcium and solubility, thereby being readily absorbed in the human body.
Soybean protein is generalized comparable to milk protein and dietetically superior and has a high economical efficiency. However, the soybean protein does not contain calcium since it cannot be phosphorylated in plants and thus cannot be a main source of calcium. Therefore, it would be significant if peptide obtained from natural soybean protein can be effectively phosphorylated to develop calcium preparations having high calcium-binding ability, solubility in water and absorption rate in the body.
In view of the circumstances as described above, the present inventors have prepared soybean phosphopeptide calcium by chemically phosphorylating isolated soybean protein and hydrolyzing the product by a hydrolase, or enzymatically hydrolyzing isolated soybean protein and phosphorylating the hydrolysate, followed by calcium-binding reaction and examined the calcium-binding ability of the soybean phosphopeptide calcium. The present invention is completed based on the above works.